In general, a hydraulic excavator which is a typical example of a construction machine is composed of an automotive lower traveling structure, an upper revolving structure rotatably mounted on the lower traveling structure, and a working mechanism provided capable of moving upward/downward on the front side of the upper revolving structure.
The upper revolving structure is composed of a revolving frame forming a support structural body, a floor member provided on the revolving frame, in which the rear side is an operator's seat mounting portion on which an operator's seat is mounted and the front side is a foot rest portion on which the operator places the foot, a cab box provided by covering the periphery and the upper part of the floor member in order to form an operator's room on the floor member, an indoor unit for an air conditioner arranged on the floor member and supplying sucked air into the cab box as conditioned air, an outdoor air inlet port provided by being opened to the outside of the cab box, and an outdoor air duct provided by connecting the outdoor air inlet port and the indoor unit and supplying the outdoor air introduced through the outdoor air inlet port into the indoor unit. On the cab box, a door to be opened/closed when the operator gets on/off is provided on the left side face.
Here, the outdoor air inlet port is located on the rear side of the cab box and provided by being opened to the outside, and an outdoor air filter for trapping dusts in the outdoor air to be introduced is provided at a position of this outdoor air inlet port. In this case, a filter cap is provided at the outdoor air inlet port for preventing removal of the outdoor air filter. In replacement of the filter, the filter cap is removed, the outdoor air filter is attached to the outdoor air inlet port, dusts are trapped by this outdoor air filter, and clean air is supplied into the indoor unit for an air conditioner (Patent Document 1).